N.sub.2 streams in chemical plants and air streams in chemical processes are contaminated by volatile organic compounds and enormous amounts of these compounds are discharged into the atmosphere. Small and mid-size distributed sources of such VOC-contaminated air are found in air stripping product streams, centrifugal purge/inerting systems, degreasing of metal parts, dry cleaning stores, printing and painting facilities, propellant manufacturing operations, soil decontamination facilities, ventilation systems, etc. The solvent encountered in such air streams are, for example, toluene, xylene, acetone, trichloroethylene, trichloroethane, methanol, ethanol, etc. These solvent vapors pose a serious environmental problem, which in turn creates a large financial expense to those companies that produce streams of volatile organic compounds. Under the Clean Air Act and government regulations, volatile organic compounds can no longer be simply discharged into the air. It is now mandatory to treat such air streams to remove volatile organic compounds.
Common methods of reducing emissions are adsorption on activated carbon, absorption in a liquid, incineration or thermal oxidation (usually without energy recovery) and catalytic oxidation. There are disadvantages to these common methods.
Firstly, adsorption on activated carbon is the most widely used process. However, this process is quite expensive, especially if the organic content in the process air stream exceeds 0.1-0.5% (Baker, R.; C. M. Bell; and H. Wijmans: "On Membrane Vapor Separation versus Carbon Absorption," Paper 174d presented at the AIChE Annual Meeting, San Francisco, Calif. (1989)). In addition, relative humidity should be lower than 30-50% for carbon adsorption to be effective. The exothermic adsorption process leads to high temperatures in the carbon beds for higher organic concentrations, resulting in persistent operational problems and even fires in the activated charcoal plant (Armand, B. L.; H. B. Uddholm; and P. T. Vikstrom: "Absorption Method to Clean Solvent-Contaminated Process Air", Ind. Eng. Chem. Res., 29, 436 (1990)). This type of method is not effective in removing light hydrocarbons. Additionally, expensive construction materials must be used to lessen contamination of activated carbon and the corrosion of equipment, which occurs during steaming to recover the solvent from the carbon bed. Many solvents hydrolyze in the presence of water or steam at high temperatures and the activated carbon acts as a catalyst for these hydrolysis reactions (Kohl, A. and F. Riesenfeld, Gas Purification 3rd Edition, Gulf Publishing Co., Houston, (1979)).
Conventional liquid absorption systems are too bulky and costly for processes with small or large air flow. For small air flow, the capital cost of air absorption apparatus is not cost effective; for large systems, the scale-up is difficult. In addition, absorption systems are subject to flooding, loading, entrainment, etc.
Incineration is an unattractive method because of the very dilute concentrations of volatile organic compounds in the air and the possibility of forming chlorinated compounds like dioxin (Armand, B. L.; H. B. Uddholm; and P. T. Vikstrom: "Absorption Method to Clean Solvent-Contaminated Process Air," Ind. Eng. Chem. Res., 29, 436 (1990)). The incineration method also requires supplemental fuel-firing unless the volatile organic compound concentration is quite high.
Another method for removing volatile organic compounds from air applies a vacuum to one side of a permselective membrane. Nonporous polymeric rubbery membranes are highly selective for volatile organic compounds (Peinemann, K. V.; J. M. Mohr; and R. W. Baker: "The Separation of Organic Vapors from Air", AIChE Symp. Ser., 82 (250), 19 (1986)). Using this method, however, it is difficult to bring the volatile organic compound concentration below 200 ppm. A further disadvantage to this method, if an air/N.sub.2 selective membrane is used, is that Air/N.sub.2 has a very low permeability through most membranes. As a result, the membrane area must be very large so as to provide adequate permeation. The cost of such a large membrane area would be very high.
A further method for removing volatile organic compounds from air uses biofilters. One known disadvantage to this method is that the microorganisms, if available, usually metabolize a specific compound or class of compounds; they cannot metabolize arbitrary volatile organic compound mixture. An enormous amount of research and development will be required to determine whether microorganisms can effectively reduce the volatile organic compounds in air to a level of 1-5 ppm.
Hollow fiber devices have been used to strip volatile species from water (Zhang, Qi and E. L. Cussler: "Microporous Hollow Fibers for Gas Absorption", J. Membrane Sci., 23 321 (1985); Semmens, M. J.; R. Qin; and A. Zander: "Using a Microporous Hollow Fiber Membrane to Separate VOCs from Water", Journal AWWA, April, 162 (1989)), or absorb gases in aqueous solutions (Zhang, Qi and E. L. Cussler: "Microporous Hollow Fibers for Gas Absorption", J. Membrane Sci., 23, 321 (1985); Karoor, S. and K. K. Sirkar: "Gas Absorption Studies in Microporous Hollow Fiber Membrane Modules", Ind. Eng. Chem. Res., 32, 674 (1993)). In conventional hollow fiber gas-liquid contactors, the pore is usually gas filled and the absorbent does not wet the hydrophobic fibers. The absorbent, in the conventional systems, is at a pressure higher than that of the gas, and the gas-liquid contacting interface at the pore mouth is on the liquid side of the fiber (Zhang, Qi and E. L. Cussler: "Microporous Hollow Fibers for Gas Absorption", J. Membrane Sci., 23, 321 (1985)).
Other devices permit nondispersive gas absorption using an aqueous nonwetting absorbent in the pores of the microporous hydrophobic fiber and the gas phase at a higher pressure (Karoor, S. and K. K. Sirkar: "Gas Absorption Studies in Microporous Hollow Fiber Membrane Modules", Ind. Eng. Chem. Res., 32, 674 (1993)). In that device, the absorbent had to be introduced by a complicated exchange process since it was nonwetting.
Other efforts using nonporous hollow fibers for nondispersive gas-liquid contacting for volatile organic compound scrubbing or removal by an organic wetting liquid have failed due to an inadequate understanding of the role of phase pressure (Jansen, A. E.; P. H. M. Feron; J. J. Akkerhuis; and B. P. T. Meulen: "Vapor Recovery from Air with Selective Membrane Absorption", Paper presented at ICOM '93, Heidelberg, Germany, Sep. 2, 1993).
The prior art includes a number of separation devices. These devices perform satisfactorily for their purpose, however, there is room for improvement.
U.S. Pat. No. 4,750,918 to Sirkar, issued Jun. 14, 1988, relates to an apparatus which permits a gas to be selectively transferred from a feed gas mixture to an output fluid. This device does not provide a means for regenerating the output fluid so as to permit its reuse.
European Patent Publication No. 0430331A1 relates to a method for removing organic compounds from air by flowing air on one side of a membrane and flowing a liquid in which the organic compounds are highly soluble in a countercurrent direction on the other side of the membrane.
U.S. Pat. No. 4,973,434 to Sirkar et al., issued Nov. 27, 1990, relates to a single-ply immobilized liquid membrane, which is immobilized within a hydrophobic microporous support, and the process for making such a membrane.
U.S. Pat. No. 4,789,468 to Sirkar, issued Dec. 6, 1988, relates to an apparatus for liquid--liquid solute-transfer. The apparatus consists of a feed solution chamber, a liquid extractant chamber, and a pressure difference regulator. In operation, the feed solution is pumped into the feed solution chamber at a substantially constant rate under pressure. The extractant is pumped into the extractant chamber at a controlled pressure. The feed solution contacts one side of the porous membrane. Pressures of the feed solution and the extractant are imposed in directions and magnitude to substantially immobilize the interface between the feed solution and the extractant at the porous membrane. The solute passes through the pores of the membrane into the extractant. The extractant is then discharged from the housing.
U.S. Pat. No. 4,921,612 to Sirkar, issued May 1, 1990, relates to an asymmetrically-wettable porous membrane and a process for transferring solute from a liquid feed solution to a liquid extractant, which is substantially immiscible with the feed solution. The housing of the unit has an asymmetrically-wettable porous membrane which divides the interior of the housing into a feed chamber, into which a feed solution is pumped then discharged, and an extractant chamber, into which an extractant is pumped then discharged. The side of the membrane facing the feed solution chamber is hydrophilic whereas the side of the membrane facing the extractant chamber is hydrophobic. Pores in the membrane permit communication between the feed solution and the extractant. The solute diffuses into the extractant. The extractant containing the solute is then discharged from the unit. This device does not provide a means for regenerating the extractant.
U.S. Pat. No. 5,053,132 to Sirkar, issued Oct. 1, 1991, is a continuation of the previously discussed patent which relates to the asymmetrically-wettable porous membrane.
U.S. Pat. No. 5,198,000 to Grasso et al., issued Mar. 30, 1993, relates to a method and apparatus for removing volatile compounds from a contaminated gas stream.
The citation of any reference herein should not be deemed an admission that such reference is available as prior art to the invention.